Sir John Major made the comments while speaking at the Irish embassy in London on Thursday.
Photograph: Will Oliver/EPA

Sir John Major has warned that a hard border in Ireland will be unavoidable unless Brexiters start to take on board the “collateral damage” that exiting the EU will cause in Northern Ireland.

He said that the words “customs union” had become “toxic” and that efforts to find an alternative solution to the border amounted to “limp promises”.

The Conservative former prime minister said that Brexiters needed to understand that unless the UK stays in a customs union and has regulatory alignment with the EU, checks will be required for food, animals and animal feed by law.

Quick guide

Why is the Irish border a stumbling block for Brexit?

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Counties and customs

Inside the EU, both Ireland and Northern Ireland are part of the single market and customs union so share the same regulations and standards, allowing a soft or invisible border between the two.

Britain’s exit from the EU – taking Northern Ireland with it – risks a return to a hard or policed border. The only way to avoid this post-Brexit is for regulations on both sides to remain more or less the same in key areas including food, animal welfare, medicines and product safety.

Early drafts of the agreement Britain hoped to get signed off on Monday said there would be “no divergence” from EU rules that “support north-south cooperation”, later changed to “continued alignment” in a formulation that appeared to allow for subtle divergences.

But it raised new questions about who would oversee it and how disputes might be resolved. It was also clearly still a step too far for the DUP.

Photograph: Design Pics Inc/Design Pics RF

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“If so, a physical border seems unavoidable,” he said. “And, since the border winds through over 300 miles of countryside, this may require a number of border posts to be erected.

“No doubt many goods can be cleared in some invisible, frictionless way – as yet unidentified – but not all. Some, such as animals and animal feed, which cross the border every hour of every day, will probably have to be examined for health and safety reasons in order to avoid infections and diseases.”

His comments in a speech at the Irish embassy in London on Thursday come weeks after he said the British people had “every right” to a second referendum arguing that voters were misled by campaigners.

Major warned that border checks had the potential to “divide communities that are now united” and provide “a focus for protests from fringe groups – either unionists or nationalists”.

Q&A

What is a customs union and why does it matter?

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A customs union is an agreement by a group of countries, such as the EU, to all apply the same tariffs on imported goods from the rest of the world and, typically, eliminate tariffs entirely for trade within the group.

By doing this, they can help avoid the need for costly and time-consuming customs checks during trade between members of the union. Asian shipping containers arriving at Felixstowe or Rotterdam, for example, need only pass through customs once before their contents head to markets all over Europe. Lorries passing between Dover and Calais avoid delay entirely.

Customs are not the only checks that count – imports are also scrutinised for conformity with trading standards regulations and security and immigration purposes – but they do play an important role in determining how much friction there is at the border.

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He said politicians should be aware of the consequence of that and ask themselves if there were protests would the border need to be protected. “Would security be brought in?” If so, would that prompt “a downward spiral towards violence”?

“I don’t wish to overplay this but, at the very least, it increases the potential for conflict,” he added.

Major was the first prime minister to normalise Anglo-Irish relations but his interventions have lead to virulent attacks by Brexiters.

Major said in his speech that when he and Tony Blair went to Ireland to warn about the implications of Brexit for the whole island he expected opposition from Brexit supporters. “What we didn’t expect was a complete failure of understanding about its impact by leading Brexiters.

Brexit plan drawn up for border checks between NI and rest of UK

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“I don’t believe for a moment that the British government wishes to hurt Ireland: far from it. I simply observe that preventing collateral damage to Irish interests doesn’t appear to feature prominently in the concerns of those who press for Brexit.”

Major conceded that the customs union does have downsides but that these are “heavily exaggerated”.

He said the Irish border question was “much more than a political squabble” between Brexiters and remainers, rather it was “a battle for the future direction of a policy that will affect all the people of the UK and the Republic of Ireland”.